1. Field of the Invention
The present invention relates generally to a semiconductor chip assembly, and more particularly to a semiconductor chip assembly in which a semiconductor chip is mechanically and electrically connected to a support circuit.
2. Description of the Related Art
Semiconductor chips have input/output pads that must be connected to external circuitry in order to function as part of an electronic system. The connection media is typically an array of metallic leads (e.g., a lead frame) or a support circuit (e.g., a substrate), although the connection can be made directly to a circuit panel (e.g., a mother board). Several connection techniques are widely used. These include wire bonding, tape automated bonding (TAB) and flip-chip bonding. Wire bonding is by far the most common. In this approach, wires are bonded, one at a time, from the chip to external circuitry by ultrasonic, thermocompression or thermosonic processes. TAB involves bonding gold-bumped pads on the chip to external circuitry on a polymer tape using thermocompression bonding. Both wire bonding and TAB require mechanical force such as pressure or a burst of ultrasonic vibration and elevated temperature to accomplish metallurgical welding between the wires or bumps and the designated surface.
Flip-chip bonding involves providing pre-formed solder bumps on the pads, flipping the chip so that the pads face down and are aligned with and contact matching bond sites, and melting the solder bumps to wet the pads and the bond sites. After the solder reflows it is cooled down and solidified to form solder joints between the pads and the bond sites. Organic conductive adhesive bumps with conductive fillers in polymer binders have been used in place of solder bumps, but they do not normally form a metallurgical interface in the classical sense. A major advantage of flip-chip bonding over wiring bonding and TAB is that it provides shorter connection paths between the chip and the external circuitry, and therefore has better electrical characteristics such as less inductive noise, cross-talk, propagation delay and waveform distortion. In addition, flip-chip bonding requires minimal mounting area and weight which results in overall cost saving since no extra packaging and less circuit board space are used.
While flip chip technology has tremendous advantages over wire bonding and TAB, its cost and technical limitations are significant. For instance, the cost of forming bumps on the pads is significant. In addition, an adhesive is normally underfilled between the chip and the support circuit to reduce stress on the solder joints due to thermal mismatch between the chip and the support circuit, and the underfilling process increases both manufacturing complexity and cost. Furthermore, the solder joints exhibit increased electrical resistance as well as cracks and voids over time due to fatigue from thermo-mechanical stresses. Finally, the solder is typically a tin-lead alloy and lead-based materials are becoming far less popular due to environmental concerns over disposing of toxic materials and leaching of toxic materials into ground water supplies.
Other techniques besides wire bonding, TAB and flip-chip bonding have been developed to connect chips to external circuitry without using wires, leads or bumps. Such techniques include thin film rerouting at the wafer, panel or module level, and attaching a pre-patterned substrate to the chip such that through-holes in the substrate expose the pads and selectively applying conductive material into the through-holes. Several approaches are described below.
A typical thin film routing approach includes depositing a dielectric material on the chip, providing through-holes in the dielectric material that expose the pads, providing metallization in the through-holes that contacts the pads, and providing a top layer of conductive circuitry on the dielectric material that contacts the metallization. In this manner, the additional circuitry is fabricated on the chip. Drawbacks to this approach include complicated manufacturing requirements, high cost, and chip loss if the additional circuitry is defective. In particular, since the chip or wafer provides a substrate for the additional circuitry, chips will be lost if the additional circuitry fails to achieve certain quality and yield criteria. Unpredictable chip loss has prevented the wide spread adoption of this xe2x80x9cchip firstxe2x80x9d approach in volume production. Furthermore, if the process is not performed on wafers, the commercially available silicon wafer processing equipment may not be compatible with common tooling and handling techniques.
U.S. Pat. No. 5,407,864 discloses providing a partially assembled printed circuit board (PCB) with buried conductive traces and through-holes that expose portions of the conductive traces, attaching the PCB to the chip using an adhesive, removing portions of the adhesive exposed by the through-holes to expose the pads, depositing a blanket conductive layer over the PCB which covers the pads and sidewalls of the through-holes without filling the through-holes, depositing a blanket insulative layer over the PCB which fills the remaining space in the through-holes, polishing the PCB to remove the conductive layer and the insulative layer from the top surface, and providing circuitry at the top surface that is connected to the conductive traces. In this manner, the circuitry at the top surface is connected to the pads through the conductive traces and portions of the conductive layer in the through-holes. Since, however, the conductive layer is blanket deposited over the entire PCB, polishing is used to remove the conductive layer from the top surface of the PCB since it would otherwise short the pads together. Polishing the conductive layer is costly and time consuming. Another drawback is that the polishing completely removes the top layer of the PCB, and therefore subsequent processes such as masking, circuitization and bumping are necessary for fabricating top surface circuitry such as traces and terminals for the next level assembly.
U.S. Pat. No. 6,037,665 discloses providing a chip with solder bumped pads, providing a pre-patterned multi-layer substrate with pre-metallized through-holes aligned with the pads, filling solder from the bumped pads into the through-holes, and reflowing the solder to form solder joint connections with the pads. This approach is similar to flip-chip bonding except that the solder is filled into the through-holes instead of merely being disposed between the chip and the substrate.
Drawbacks to this approach include the need to solder bump the chip as well as the disadvantages of solder joints discussed above.
U.S. Pat. No. 5,116,463 discloses attaching a multi-layer substrate to a chip that includes forming through-holes through a dielectric layer that extend to the pads and electrolessly depositing metal into the through-holes. The electroless deposition is initiated by the pads and continues until the deposited metal fills the through-holes and contacts metallization on the top surface of the substrate. Drawbacks to this approach include the need for the metallization on the top sure to provide endpoint detection and the possibility that electroless deposition on the metallization on the top surface adjacent to the top of the through-hole may close the through-hole before the electroless deposition fills the through-hole. Furthermore, avoiding such premature closing of the through-hole imposes design constraints on the diameter of the through-hole and the thickness of the substrate which may be burdensome in some applications.
U.S. Pat. No. 5,556,810 discloses inner leads laminated by an organic film and attached to a chip by an adhesive. Distal ends of the inner leads are positioned near the pads and then electrically connected to the pads by L-shaped electroplated metal. However, since the inner leads are flexible and vary in height and length, the inner leads may not be positioned precisely and uniformly, the gaps between the distal ends and the respective pads can vary, and consequently the electroplated joints may be weak or open. Furthermore, if the chip has moderate to high pad density and a separate power/ground plane is needed to achieve better electrical performance, the single layer inner leads may not be suitable. In addition, handling of this leaded-chip for the next level assembly such as outer lead bonding or board level assembly can be problematic since the leads are soft and easily bent, rendering it difficult to maintain co-planarity among the leads during the next level assembly.
U.S. Pat. No. 5,665,652 addresses the gap variations between the inner leads and the pads by positioning the inner leads at designated locations before the electroplating starts. In this approach, columnar leads are inserted into guide holes of a guide tool on the surface of the chip, and each guide hole is aligned with one of the pads. The chip and guide tool are then soaked in an electrolytic solution during electroplating. Since the inner leads can be brought into reliable contact with the pads during electroplating, control of the electroplated connections is improved. However, a drawback of this approach is the difficult and time consuming nature of mechanically attaching and aligning the guide tool to chip as well as maintaining its precisely-aligned position throughout the electroplating process. The gap between the guide tool and the chip needs to be rigidly maintained. If the gap is too large, the guided inner leads may tilt and bend; if the gap is too small, insufficient electrolytic solution can lead to electrical field shielding and inadequate electroplating. Furthermore, since the guide tool is temporarily attached to the chip, it must be subsequently detached from the chip leaving the inner leads standing free. As a result, board level assembly of this leaded-chip can have high yield loss.
Recent introduction of grid array packaging (e.g., ball grid arrays), chip size packages (CSP) and flip-chip packages using high density interconnect substrates are relentlessly driving increased printed circuit board density. Shrinking traces and spaces and increasing layer count increase printed circuit board density, however reducing the size of plated through-holes can even more significantly increase printed circuit board density. Small through-holes allow more routing space so that more conductive lines can be placed between the through-holes. Small through-holes also increase design flexibility and reduce design cycle time and overall product introduction time.
Conventional printed circuit boards have drilled through-holes with a size (diameter) in the range of 200 to 400 microns. As drilling technology improves, the drilled through-hole size is anticipated to reach 100 microns. Moreover, recently developed methods for forming through-holes using a punch, plasma or laser have driven down through-hole size to the range of 50 microns or less. A typical chip pad has a length and width on the order of 50 to 100 microns. Since the through-holes allow the pads to interconnect with various circuitry layers, using through-holes with similar sizes to the pads is desirable. The major advantage of using metallization in through-holes to interconnect the pads is that it replaces external media such as wires, bumps and leads.
The semiconductor chip assembly is subsequently connected to another circuit such as a PCB or mother board during next level assembly. Different semiconductor assemblies are connected to the next level assembly in different ways. For instance, ball grid array (BGA) packages contain an array of solder balls, and land grid array (LGA) packages contain an array of metal pads that receive corresponding solder traces on the PCB. However, since BGA and LGA packages are connected to the PCB by solder joints, the compliance is small and solder joint reliability problems exist. BGA and LGA packages have been designed with pillar post type contact terminals that extend above the package and act as a stand-off or spacer between the package and the PCB in order to absorb thermal stresses and reduce solder joint fatigue. Plastic quad flat pack (PQFP) packages have a lead frame formed into a gull-wing shape. When the PQFP is assembled on a PCB, this gull-wing lead serves as the contact terminal which provides compliance and reduces stress on the solder joints. However, drawbacks to PQFP packages include the large size of the lead and poor high frequency electrical characteristics.
In view of the various development stages and limitations in currently available semiconductor chip assemblies, there is a need for a semiconductor chip assembly that is cost-effective, reliable, manufacturable, provides excellent mechanical and electrical performance, and complies with stringent environmental standards.
An object of the present invention is to provide a semiconductor chip assembly with a chip and a support circuit that provides a low cost, high performance, high reliability package.
Another objective of the present invention is to provide a convenient, cost-effective method for manufacturing semiconductor chip assemblies as chip size packages, ball grid arrays or other structures.
In accordance with one aspect of the invention, a method of manufacturing a semiconductor chip assembly includes providing a semiconductor chip that includes a conductive pad, providing a support circuit that includes an insulative base, a conductive trace and a through-hole that extends through the conductive trace, mechanically attaching the chip to the support circuit such that the insulative base covers the through-hole and a portion of the pad is directly beneath the through-hole, and then applying an etch to form an opening in the insulative base that exposes the through-hole.
In accordance with another aspect of the invention, a method of manufacturing a semiconductor chip assembly includes providing a semiconductor chip that includes a conductive pad, providing a support circuit that includes an insulative base, a conductive trace, and a through-hole that extends through the conductive trace, wherein the insulative base covers a top of the through-hole, providing an adhesive that covers a bottom of the through-hole, then applying an etch to form an opening in the base that exposes the top of the through-hole, applying an etch to form an opening in the adhesive that exposes the bottom of the through-hole, and then mechanically attaching the chip to the support circuit using the adhesive such that the openings and the through-hole expose the pad.
Preferably, the method includes forming a connection joint inside the through-hole that extends through the opening in the adhesive and electrically connects the conductive trace and the pad.
It is also preferred that the conductive trace includes a pillar and a routing line, the pillar extends above the base and the routing line, the routing line extends below the base, and the through-hole extends through the routing line.
An advantage of the present invention is that the semiconductor chip assembly need not include wire bonds, TAB leads or solder joints. Another advantage is that the assembly can be manufactured using low temperature processes which reduces stress and improves reliability. A further advantage is that the assembly can be manufactured using well-controlled wet chemical processes which can be easily implemented by circuit board, lead frame and tape manufacturers. Still another advantage is that the assembly can be manufactured using materials that are compatible with copper chip and lead-free environmental requirements.
These and other objects, features and advantages of the invention will be further described and more readily apparent from a review of the detailed description of the preferred embodiments which follows.